Depending on the supplier, the build of the doll will be either a fixed head or a removable head.
Removable heads have creases in the neck where the doll head attaches to the body. This crease is hardly noticeable and will not affect realism. The advantage of removable heads is that you can buy more than one head for the same body and inter-change the heads which allow you to have some form of variety with your sex doll. It also allows you to purchase additional heads online without having to purchase more bodies.
On the other hand, fixed heads are creaseless and the most realistic build for sex dolls. You can interchange heads before purchasing but you will not be able to change the head once the doll is built. The advantage of fixed heads is mostly for suppliers, fixed head builds are harder to replicate which decrease the chances of fraudulent suppliers from attempting to steal their designs.
Brands with removable head designs:
6YE Premium
HR Doll
WM Doll
EXDOLL
ILDoll
JYDoll
Brand with fixed head designs:
Piper Doll
Doll-Forever
Doll House 168